The present invention relates broadly to a modulation detection apparatus, and in particular to a large dynamic range, low distortion amplitude modulation detector apparatus.
In the prior art, amplitude modulation envelope detectors have been employed in electronic systems to obtain video bandwidth information frequencies as a linear function of the modulated carrier signal. Since the modulation bandwidth may be on the order of Mega-Hertz and in order to obtain the modulation waveform at the output of the detector with least distortion, the impedance which is acting as a load, is generally kept low to provide a wide video bandwidth. However, for small load impedances, low amplitude input signals to the detector become affected by the non-linear region of the operating characteristics of the detector components. Increasing the envelope detector drive level to remain above the component non-linear operating region creates the disadvantage of requiring high drive power requirements for the detector. Therefore, forward biasing of the detector components has been employed to improve the linearity of component operating characteristics at low input levels. However, the temperature drift component characteristics of these forward biased detectors resulted in an unstable offset voltage causing the loss of low level input signals.
An additional problem with the prior art detectors is the inability to accurately reproduce the envelope of the IF signal input. Particularly, when the rate of decrease of the IF envelope is much greater than the time constant of the envelope detector, the output from the detector will only be an approximation of the actually IF input signal. This is commonly referred to as diagonal clipping. Such diagonal clipping is particularly undesirable in high resolution radar application. Such behavior leads to modification of the waveform resulting from fluctuating large signals such as sea clutter in that fluctuations appear in range cells at the receiver output and are not present in those range cells in the IF signal of the receiver. A form of cross modulation, therefore, exists which degrades detection of desired signals against a clutter background.